If this AP is getting an IP Address from a DHCP (same range as all other devices) then you should be able to find it using tools such as Angry IP scanner. All you need to do is scan all the IPs on the subnet with this.
If this AP is acting as a router and providing the DHCP services for clients connecting to it, then it should be easy, as it would be reflected in the client's configuration. You can run IPconfig/all on the client machine to view all the details of the DHCP server and default gateway IP address information.
Depending on the model and vendor of the AP you can always find the default IP address, as they are standard.
Alternatively scan for all active HTTP Port 80 on the subnet. Since the Web interface of the AP runs on Port 80 (unless changed to some other port,) you should be able to identify the same. If not, do a port scan for all the ports on the subnet using a port scanner.
This was first published in July 2008